The object of this research project is to develop a better understanding of the relationship between membrane structural features and the processes which regulate transport in epithelia. Experiments are planned to evaluate the role in the action of vasopressin of cytoplasmic vacuoles with intramembrane particle aggregates. Additional freeze-fracture and thin section electron microscope studies will examine the effect of varying the potassium content of the diet and steroid hormone treatment on membrane structural features of the distal nephron.